1. Field of the Invention
The present invention involves an improved nose cone structure and seeker housing for an aircraft.
2. Background of the Prior Art
In a variety of aircraft, the optical window for seeker systems have been placed in the nose of the aircraft. A conventional convex nose placed in a supersonic air stream is enveloped by a shock wave lying close to the nose cone surface. Within the shock layer, the temperature and pressure of the gases increase to extremely high values because of the resulting high compression ahead of the nose body. For example, temperatures reach as high as 14,000.degree. R behind a mach 16 flow at a 60,000 foot altitude. The resulting flowfield, in turn, creates substantial aberrations in optical transmission/reception, increased complexity to the design of the optical system and decreased targetting accuracy.
In a side-looking window on an aircraft with a conventional nose cone, FIG. 1, flying at hypersonic speeds window well 12 is subjected to the supersonic gas flow. In order the overcome the temperature constraints, side-looking optical windows have required window cooling equipment to reduce aero-optical effects produced by the high temperatures. Any inclusion of a cooling system, however, necessarily adds expense, weight and increased complexity. The flow about the window well 12 consists of several layers including an outer undisturbed shock layer 16, a mixing layer 18, a window coolant boundary layer 19 and a window shock 22. The interaction between these layers creates a complex flow field which, in turn, impacts on optical signal transmission.
As discussed, placement of the window along the side of the nose reduces optical signal quality. Depending on the optical system, an optical signal may make a single or double pass through a complex flow field composed of a curved shock layer, a coolant mixing layer, and a transition or turbulent boundary layer. The aero-optical effects resulting from such a passage may include bore sight error, image blur or distortion, beam divergence, scintillation, absorption and unpredictable fluctuations of one or more of these features. The optical signal, for example, can be subject to unsteady density fluctuations in the window field of view such that the random wave front errors reduce signal resolution. The curved density contours along the shock layer can also produce wave front astigmatisms and higher order aberrations distorting the optical image. Turbulence along the boundary and mixing layers of the window cooling system can also introduce image blur, beam divergence and significant signal scintillation. Temperature also impacts on optical quality where ablation products can change the mean radiance and transmittance of the flow field and, combined with the aforementioned turbulence, results in fluctuating absorption of the optical signal.
The loss of efficiency, therefore, entails expensive alternatives. The loss can have other impacts. In missile applications, for example, the use of warheads becomes essential where an aircraft guidance system cannot be depended upon to actually contact the target.
Conventional aeronautic nose cone designs for front or side-looking seeker windows, therefore, have substantial drawbacks for optimizing heat, pressure and optical transmission about the optical window.